Materials which emit light when an electric current is passed through them are well known and used in a wide range of display applications. Liquid crystal devices and devices which are based on inorganic semiconductor systems are widely used, however these suffer from the disadvantages of high energy consumption, high cost of manufacture, low quantum efficiency and the inability to make flat panel displays.
Organic polymers have been proposed as useful in electroluminescent devices, but it is not possible to obtain pure colours, they are expensive to make and have a relatively low efficiency.
Another compound which has been proposed is aluminium quinolate, but this requires dopants to be used to obtain a range of colours and has a relatively low efficiency.
In an article in Chemistry letters pp 657-660, 1990 Kido et al disclosed that a terbium III acetyl acetonate complex was green electroluminescent and in an article in Applied Physics letters 65 (17) 24 October 1994 Kido et al disclosed that a europium III triphenylene diamine complexes was red electroluminescent but these were unstable in atmospheric conditions and difficult to produce as films.
The complexes disclosed in these articles had a low photoluminescent efficiency and were only able to produce green or red light and other colours could not be produced.
We have now discovered electroluminescent materials which have a higher photoluminescent efficiency and which can produce a range of colours, which has hitherto been difficult to produce.
The higher photoluminescent efficiency enables these materials to be used in a range of applications.
According to the invention there is provided an electroluminescent device comprising a transparent substrate on which is formed a layer of an electroluminescent material in which the electroluminescent material is a rare earth metal, actinide or transition metal organic complex which has a photoluminescent efficiency (PL) of greater than 25%.
The photoluminescent efficiency is a measure of the efficiency of conversion of absorbed light to emitted light and can be measured as described in the Article in Chemical Physics Letters 241 (1995)89-96 by N. C. Greenham et al.